Sorry not really, I guess I'm just trying to understand the actual use case. What exactly is it you want, say a user or someone calling one endpoint to do? Can you describe a user story? What will trigger one endpoint to need to call another without it being initiated on the client side?
Lambda functions are pretty self contained, the serverless.yml really stops at your deployment unless you want to inject stuff into the environment variables of the function. Is there any reason you would be calling the other functions through the API gateway, rather than just invoking them inside the function you want to call them with?
If you just invoke the function directly inside the calling function, you'll cut out a lot of API gateway overhead. Also with serverless using a naming convention approach, you won't need to know the path, as you can just use the function name.
The other option would be to include your serverless.yml file in your lambda function so you could read it into an object inside the function at runtime and reference whatever config items you need? Feels a bit hacky though